The known technique for detecting misfiring is primarily based on two different methods, namely, measuring the rotational speed of the flywheel or crankshaft of the internal combustion engine, or measuring the torque of the crankshaft of the internal combustion engine.
The most commonly used method is based on measurement of the rotational speed of the flywheel. This measurement occurs either instantaneously with the aid of some kind of analog speed sensor, or as a mean value; in which case the measurement is normally digital. Then, an analysis is made of the very small differences in the rotational speed which arise, especially during misfiring, in order to determine whether or not misfiring has occurred. From a purely general point of view, this method is very sensitive to measurement errors and does not function satisfactorily for all loads and speeds.
An embodiment comprising the above method is described in U.S. Pat. No. 5,239,473 "Method and system for detecting the misfire of an internal combustion engine utilizing angular velocity fluctuations". In this case, the rotational speed of the crankshaft and electronic signal processing are utilized. After the speed signal has been processed to reduce random faults and external disturbances, the signal may be utilized in four different described methods of calculation to obtain information as regards the torque developed in each cylinder. This results in a vector with M dimensions for the behaviour of each cylinder for each cycle of the internal combustion engine. Here, M represents the number of cylinders of the engine. By analysis of this vector with the aid of statistical decision theory, it is possible, according to the patent specification, to obtain information about any misfire.
The other method, as mentioned above, comprises directly measuring the torque in the crankshaft. The principle as such functions very well for detecting misfiring. However, the problem is that the application of a torque transducer requires quite an extensive intervention in an existing engine design and is, therefore, not particularly attractive to the automobile industry.
An embodiment comprising this method is described in WO 92/10733 A2, "Engine misfire, knock or roughness detection method and apparatus". The method comprises determining, at regular intervals, the mean and standard deviation of a number of signals originating from torsional stresses of the crankshaft, determining the difference in mean value between consecutive measurements, determining the ratio between this difference and the standard deviation and then comparing the difference against a threshold value. This ratio for a given cylinder is then compared with one or more corresponding ratios for other cylinders in the internal combustion engine. It is further clear from the patent document that magneto-strictive sensors may be used to obtain a measure of the torsional stresses.
As will have been clear from the above description, misfiring of internal combustion engines is detected on the basis of measurement of the rotational speed of the crankshaft or the torque developed in the crankshaft and by means of advanced and extensive analytical and statistical methods. It has also been mentioned that there are problems in connection with the sensitivity to measurement errors when analyzing speed signals and in connection with the application of current measuring transducers and, in particular, with the application of torque transducers.